Cornball chemistry

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Monthly Archives: January 2015

SAFETY FIRST: This demonstration must be done with care and adequate ear protection must be worn by all present, along with safety spectacles. The use of commercial ear protectors are advised.

A propane rocket

One demonstration that can be used to show how the ratio of reacting gas volumes relates directly to mole quantities in a balanced chemical equation is with a “propane rocket”.

In the experiments described below we used bottled gas, or LPG, which contains a mixture of propane and butane, in a 600 ml plastic bottle. Oxygen was also required. When ready to launch the bottle rocket was suspended from a fishing line using two paper clips and two rubber bands.

Fuelled and ready to launch

The balanced equations for the complete combustion of propane and butane are shown below.

Equations for the complete combustion of propane and butane

Thus, it can be seen from the first equation that 5 volumes of oxygen are required to completely burn 1 volume of propane.

The gases are introduced into the bottle rocket by displacing water as shown. It is recommended that a fresh plastic bottle is used each time.

Oxygen from a gas syringe

Followed by LPG

When the gases are used in the proportion of 5 volumes of oxygen to 1 volume of bottled gas, a very loud explosion propels the bottle at speed across the laboratory.

Complete combustion.. BANG!

Reduce the proportion of oxygen and incomplete combustion occurs. By trial and error a slower flying rocket can be obtained.

With 3 to 4 volumes of oxygen : 1 volume of propane a slower rocket results

However, if the proportion of oxygen is reduced too far the rocket will fail to launch.

Too much fuel, not enough oxygen – disaster!

All of the above .gif animations were produced from high speed movies shot at 120 fps (frames per second). Here is another animation produced from a movie shot at 1000fps

High speed movie 1000fps

Frame by frame

Thus, the effect of varying the molar ratio of the two reacting gases, using reacting gas volumes, can be shown.